This invention relates to electric detonators generally and more particularly to a static electricty suppression arrangement for use in two-wire electric detonators.
Large explosive charges are detonated by initiating devices or detonators which are of two types--electric or nonelectric. An electric detonator (blasting cap) converts electrical energy into heat energy which, in turn, produces an explosive force capable of detonating a large explosive charge. The electrical energy is supplied to the detonator by two electrical conductors, called leg wires, which typically enter the detonator through a rubber or plastic sealing plug. The ends of the leg wires inside the detonator are joined together by a high resistant "bridge wire" which, when sufficient current flows through it, heats up to ignite a heat sensitive material which surrounds the bridge wire. This, in turn, ignites delay fuse elements to thus ignite or detonate a primary explosive charge which then detonates a base explosive charge. The explosive force developed by the base explosive charge is used to detonate the aforementioned large explosive charge.
The explosive charge's delay fuse elements, heat sensitive material, and sealing plug are encased in a cylindrical shell made of an electrically conductive material such as aluminum, bronze, etc. The plug is positioned in one end of the shell to hold the leg wires in positions spaced from the shell wall, and to guide the leg wires to the heat sensitive material.
Problems with electric detonators include static charge build-up on the leg wires, and static charge sources external to the detonator which, when in close proximity with the leg wires, cause current to flow through the leg wires and detonator to ground. Discharge of such static electricity through the bridge wire or the heat sensitive material can cause accidental/premature detonation and result in serious injury to users. Conventional methods of dealing with static electricity generally involve the provision of a discharge path from each leg wire to the electrically conductive shell. The idea behind this is that if the static electricity can be "routed" around the bridge wire and explosive train via the shell, then dangerous premature detonation, at least that caused by static electricity, may be avoided. A problem and drawback of this approach is that slight differences in voltage breakdown between the two discharge paths can cause electrical current to flow through the bridge wire to prematurely initiate detonation.